One of the key characteristics of the immune system is the ability to react to pathogens by generating antibodies. An effective immune response requires the production of antibodies that bind antigens with high affinity and specificity. Exposure to antigen triggers the generation and clonal selection of B cells carrying novel mutant Ig sequences with improved antigen affinity, in a phenomenon known as affinity maturation. Affinity maturation is the result of the combination of two processes: somatic hypermutation and affinity-based selection, both of which occur in anatomic structures referred to as germinal centers. Recent studies suggest that the antigen-dependent interaction between B cells at germinal centers and follicular T helper (Tfh) cells, which are the limiting factor in affinity-based selection of B cells that express high affinity antibodies. According to the proposed model, B cells that exhibit high affinity Ig molecules at the plasma membrane will capture and process more antigen for presentation on major histocompatibility complex (MHC) class II molecules. A limited number of Tfh cells then selects those B cells with the highest peptide-MHC density and directs their return to the dark zones in germinal centers, where the selected B cells undergo rapid division. By contrast, B cells that fail to interact with Tfh cells undergo apoptosis.
Despite the crucial role of the interaction between Tfh cells and B cells in affinity maturation, little is known about how these interactions lead to selection of some cells and elimination of others in an in vivo setting. This gap is due largely to the fact that there is no effective way to determine the extent to which two cells have interacted within a living animal. Interactions between different ligand-receptor pairs expressed by various subsets of immune cells are key events in the immune response, but the tracking of these interactions in the context of a living animal has never been achieved.